The present invention concerns electrolytic capacitors, particularly those for use in medical devices, such as implantable defibrillators.
Every year more than half a million people in the United States suffer from heart attacks, more precisely cardiac arrests. Many of these cardiac arrests stem from the heart chaotically twitching, or fibrillating, and thus failing to rhythmically expand and contract as necessary to pump blood. Fibrillation can cause complete loss of cardiac function and death within minutes. To restore normal heart contraction and expansion, paramedics and other medical workers use a device, called a defibrillator, to electrically shock a fibrillating heart.
Since the early 1980s, thousands of patients prone to fibrillation episodes have had miniature defibrillators implanted in their bodies, typically in the left breast region above the heart. These implantable defibrillators detect onset of fibrillation and automatically shock the heart, restoring normal heart function without human intervention. A typical implantable defibrillator includes a set of electrical leads, which extend from a sealed housing into the heart of a patient after implantation. Within the housing are a battery for supplying power, heart-monitoring circuitry for detecting fibrillation, and a capacitor for storing and delivering a burst of electric charge through the leads to the heart.
The capacitor is typically an aluminum electrolytic capacitor, which usually includes a sandwich-like assembly of two strips of aluminum foil with two strips of paper, known as separators, between them. One of the aluminum foils serves as a cathode (negative) foil, and the other serves as an anode (positive) foil. Sometimes, two foils are stacked one on the other to form a dual anode. Attached to each foil is an aluminum tab which electrically connects the foil to other parts of the capacitor.
The foil-and-paper assembly, known as an active element, is then placed in a case, usually made of aluminum, and the paper is soaked, or impregnated, with a liquid electrolyte—a very electrically conductive solution containing free positive or negative ions. After the paper is impregnated, the case is sealed shut with a lid called a header. Extending from the header are two terminals connected respectively to the anode foil and cathode foil via the aluminum tabs.
In recent years, manufacturers of aluminum electrolytic capacitors have improved capacitor performance through the development of aluminum foils with increased surface areas. Increasing surface area of a foil, particularly the anode foil, increases capacitance and thus the charge-storage capacity of a capacitor.
One approach to increasing surface area of a foil is to chemically etch microscopic hills and valleys into both sides of the foil. The etching depth is controlled to leave a solid core layer between the sides of the foil. Thus, foils with this type of etching are called “core etched.” Although core-etched foils have more surface area, they don't work as well as expected in capacitors with two stacked anode foils, because the solid core layer of one anode foil shields the other anode foil from electrolyte flow.
Another approach, known as tunnel etching, entails etching both sides of a foil to form millions of tiny holes, or tunnels, completely through the foil, from one side to the other. The tunnels, which typically have an approximately circular cross-section about one-micron in diameter, allows electrolyte to flow through the foil. Thus, tunnel-etched foils overcome the electrolyte-flow problem of core-etched foils.
However, tunnel-etched foils not only have less surface area than core-etched foils but are also quite brittle and tend to break easily, particularly when rolling or winding the foils to form cylindrical capacitors. Accordingly, there remains a need to develop more durable foil structures.